The present invention relates to pumps and, in particular, to pumps for pumping molten metal.
A number of submersible molten metal pumps are known in the art, such as that shown in U.S. Pat. No. 2,948,524 to Sweeney et al. Such pumps have been provided for pumping molten metal for various purposes. Great difficulties have been encountered due to the extremely hostile environment of a molten metal bath. The parts exposed to contact with the molten metal must resist this environment. At the metal surface in a molten metal bath of aluminum, for example, the molten metal itself is corrosive, abrasive drosses are present, and the atmosphere immediately above the metal surface is oxidizing. Additionally, splashes and splatters from the bath can cause difficulties and problems on the surfaces of the pump above the molten metal surface. Consequently, materials selection and design are extremely important for a submersible molten metal pump.
In the field of working with molten metals such as aluminum, three basic different types of centrifugal pumps are utilized. First of these is a circulation pump. Circulation pumps are used to equalize temperature and alloy mix throughout a metal bath and circulate a hot metal around the bath. Most often, they are used in conjunction with a reverbatory furnace having an external well. The well is usually an extension of the charging well where scrap metal is charged. A bridge-wall is installed to isolate and protect the pump in the external well from damage by direct contact with the charged material. The circulation pump circulates the hot metal around the cold charged scrap dispersing the heat transfer insulating film surrounding the cold scrap thus improving the melt rate of scrap. The circulation also promotes homogeneity in the alloy composition in the bath. With the circulation by such a pump, the surface temperature of the bath is reduced thereby increasing the temperature differential for radiant heat transfer in the furnace thus reducing fuel consumption in the furnace and ultimately increasing production.
A second type of pump is a transfer pump in which the molten metal is taken from the external well of the furnace and transferred to a different location such as a ladle or another furnace.
A third type of molten metal pump is known as a gas-injection pump. Gas-injection pumps circulate the molten metal while adding a gas at the exit of the pump in order to de-mag the molten metal or degas the material. In the purification of molten metals, particularly aluminum, it is frequently desired to remove dissolved gases such as hydrogen, or dissolved metals, such as magnesium. The removing of dissolved gas is known as "degassing" while the removal of magnesium is known as "demagging". Demagging uses chlorine to form magnesium chloride MgCl.sub.2. Degassing can use argon (best) or nitrogen (less expensive) to attract the hydrogen in the metal. Both processes require a gas (chlorine, argon, or nitrogen) to be introduced into the molten metal. Generally, the gas is introduced through a gas tube or pipe directly into the base or casing of the pump. If the pump flow rate (metal velocity) is insufficient during demagging, the resultant magnesium chloride is not carried away fast enough and clogs the pump. There can also be a problem if the gas flow rate of chlorine is too high wherein the chlorine will blow a hole through the bottom of the base. In the prior art such as in U.S. Pat. Nos. 4,052,199, 4,169,584, 4,351,514, the gas is introduced into the metal as it is entering or exiting the molten metal pump.
As an example of circulation pumps, reference should be made to U.S. Pat. Nos. 3,984,234, 3,759,635.
As examples of transfer pumps, reference should be made to the above-noted Sweeney et al patent together with U.S. Pat. Nos. 3,048,384 to Sweeney et al, 3,092,030 to Wunder, 3,255,702 to Gehrm, 3,836,280 to Koch, and 4,786,230 to Thut.
In the area of centrifugal pumps, the pump generally includes a casing having a pump chamber and an impeller in the chamber. As is well known in the art, the pump can be designed to be a single suction pump in which case the material to be pumped enters through a single inlet generally in parallel with the pump shaft or can be a double suction pump in which two inlets are provided generally both in line with the pump shaft. The pump chamber in the casing generally defines a volute which is defined for the purpose of this application and as known in the art as a spiral casing for a centrifugal pump with an increasing cross sectional area viewed circumferentially as the outlet of the pump is approached. With the exception of U.S. Pat. No. 3,092,030 to Wunder and U.S. Pat. No. 3,984,234 to Claxon et al, all of these centrifugal pumps noted in the patents above are volute pumps.
In addition to the hostile environment at the interface between the molten metal and atmosphere, even the molten metal bath itself is not homogeneous. That is, certain suspended solids can be present including unmelted chunks of scrap metal, chunks of alloying metals, and contaminants such as refractory brick spalled from the wall of the furnace, chunks of cement, insoluble metal oxide accretions and the like. If such suspended solids of sufficient size enter the inlet of the pump, and are not immediately expelled out of the discharge, they can be caught at the lip of the volute and jam the pump. This leads to breakage of the shaft and/or destruction of the volute. Sometimes the impeller is also destroyed.
In attempts to eliminate or minimize such problems in the past, simple strainers and filters have been applied to the inlet of the pump. Recently, filters have been cemented to the bottom of the base of bottom-feed pumps. It is readily apparent that the same are unsuitable since they rapidly can clog thereby requiring cleaning. A so called "deflector disk" has been mounted for rotation such that its periphery cooperates with the inlet opening of the pump body to define an entrance passageway of suitable dimensions for restriction of admission of solid or semi-solid materials. Such construction adds one more piece to the assembly of the pump.
Additionally, it should be noted that all of the immersed impellers of the pumps of the above-noted patents are either a cup shaped centrifugal impeller having plural radial or angularly directed radial passages with a hollow center portion receiving the molten metal from the inlet and, by centrifugal action, directing the molten metal out the angular radial passages or a vaned impeller having a generally disk shaped web with flat surfaced or curved outwardly radially extending vanes.
Generally, for molten aluminum pumps,.the pump casing and the impeller are made of graphite. Metal pump parts are unsatisfactory since relatively high temperature melting metal, such as iron are dissolved when in contact with molten aluminum, in spite of the fact that molten aluminum may be at a temperature of approximately one-half of that at which iron melts. In such a situation, iron is introduced into the molten aluminum and is considered to be a contaminant.
Generally, in the prior art, the pump casing is connected to a superstructure which is positioned above the interface between the molten metal and atmosphere. Generally, one or more support posts are attached to the casing and extend upwardly therefrom and are connected to the superstructure. Generally, the support posts are slip fitted into blind bores into the pump casing and are secured there by a coating of a refractory cement or adhesive. The support post may be threadingly engaged with the casing with the addition of the cement. During the initial assembly and subsequent rebuilds, the pump must be built on a close tolerance steel alignment fixture. Cement must be manually applied (usually actually using fingers) and is often in uneven amounts under the posts creating different post heights.
At the upper ends, the support posts are generally provided with a coaxial threaded bore. The support post is slipped into a support post sleeve extending downwardly from the superstructure which is packed with a suitable ceramic cement or furnace cement and a bolt is threaded downwardly through the superstructure into the threaded bore at the top of the support post. Problems are encountered due to such structures when it is desired to replace these support post. If the interconnection between the support post and the pump casing is by threading or by conventional blind hole (typically not threaded), the post has to be cut off at the top of the casing and very careful, difficult, labor-intensive manual hammer and chisel work is required to remove the parts of the old post. At the upper end of the post and superstructure connection, the post has to be cut off just below the sleeve and careful, labor-intensive manual hammer and chisel work is required to remove all of the old post from the sleeve. Because the sleeve depends downwardly from the superstructure cutting off the connection of the support post with the sleeve is subject to any splashing of the molten metal which further complicates the replacement of the support post.
A motor is mounted on top of the superstructure and generally is an air driven pneumatic motor requiring a specific air line and air control valve. In the past, an air motor is preferable because of the extremely harsh environment. Air motors are known to be inefficient and expensive. Additionally, the use of an air motor necessarily requires an air compressor which further adds to maintenance costs and requirements. While the use of a hydraulic motor is certainly possible, considerable danger is present due to the inflammability of hydraulic fluids if there should be a leak. Hydraulic motors are also inherent inefficient and they require a separate hydraulic pump to supply the energy to run them. Further, connection and disconnection of the motor (required when mounting and demounting the pump) would inevitably result in some spillage of hydraulic fluid. It has been thought that conventional electric motors are unsuitable for the environment due to the metal dust, oxidizing atmosphere, extreme heat, and other hostile factors. Specially built electric motors have been used successfully but greatly increase the expense and cost of the pump.
Various possible mechanisms have been provided in the prior art for coupling the shaft of the motor to the shaft of the pump. These include a bayonet connection fixed to a universal joint, a straight pair of coaxial threaded joints with a universal joint therebetween, and the like. At the opposite end of the pump shaft, generally the connection between pump shaft and impeller has been by male threading on the shaft engaging female threading in the impeller with a distinct shoulder at the junction. Potential problems upon attempted replacement of the shaft or the impeller are encountered similar to the replacement problems with respect to the support post. That is, careful, difficult, labor-intensive manual hammer and chisel work is required to remove all of the old pieces.